1. Field of the Invention
The present invention relates to an ignition coil used in, for example, internal combustion engines such as automobile engines and the like, and more specifically, to an ignition coil for internal combustion engines which prevents faulty operation and the like of other circuit devices caused by the superposition of a capacitive discharge current (noise signal) flowing through an ignition plug.
2. Description of the Related Art
FIG. 9 is a arrangement diagram showing a conventional ignition coil for an internal combustion engine together with its associated circuits.
In FIG. 9, an ignition coil 4 is composed of a primary coil 1, a secondary coil 2 magnetically coupled with the primary coil 1 and a core 3 magnetically coupled with the primary coil 1 and secondary coil 2. A capacitive coupling component C4 is formed between the primary coil 1 and the secondary coil 2.
An ignition plug 5 is connected to one end of the secondary coil 2 to which a secondary voltage V2 output from the secondary coil 2 is applied. The ignition plug 5 is composed of a discharge gap having the other end grounded and arranged such that when the insulation thereof is broken, the ignition plug 5 generates discharge spark to flow discharge current i2.
A power transistor 6 has a collector connected to one end of the primary coil 1 and constitutes an interrupting circuit 7 for interrupting the feed of a primary current i1 flowing to the primary coil 1. The emitter of the power transistor 6 is grounded and a capacitive coupling component C7 is formed between the collector and the base thereof.
A battery power unit 9 is connected to the common input terminal of the ignition coil 4 and feeds the primary current i1 through the collector and emitter of the power transistor 6. An electronic control unit (ECU) 10 composed of a microcomputer applies an ignition signal G to the base of the power transistor 6 to feed and shut off a current to and from the power transistor 6.
FIG. 10 is a cross sectional view showing a specific structure of the ignition coil 4 in FIG. 9.
In FIG. 10, the primary coil 1 is composed of a wire wound around a first non-magnetic bobbin 11 and the secondary coil 2 is composed of a wire wound around a second non-magnetic bobbin 12. The primary coil 1 and first bobbin are inserted into the cavity of the second bobbin 2 and further the magnetic core 3 is inserted into the cavity of the first bobbin
The common input terminal of the ignition coil 4 and the output terminal of the primary coil 1 are connected to a connector 14 through a terminal 13 shown by a single line for convenience (actually two lines) and electrically connected to the anode of the battery power unit 9 and the collector of the power transistor 6. Further, the one end of the secondary coil 2 or the output terminal of the ignition coil 4 is connected to the connector 14 through a terminal 23 and electrically connected to an external circuit or the ignition plug 5.
Next, operation of the conventional ignition coil for internal combustion engine shown in FIGS. 9 and 10 will be described with reference to the waveform diagram of FIG. 11. FIG. 11 shows the changes in time of the respective signal waveforms of the primary current i1 (FIG. 11b) flowing in response to the ignition signal G, (FIG. 11a) the secondary voltage V2 (FIG. 11c) generated in response to the feed and shut-off of the primary current i1, and the discharge current i2 (FIG. 11d) flowing in response to the secondary voltage V2. The discharge current i2 is composed of a capacitive discharge current iC and inductive discharge current iL.
First, the power transistor 6 constituting the interrupting circuit 7 is turned on in response to the ignition signal G (power transistor drive signal) of a high level output from the ECU 10 and starts flowing the primary current i1 to the primary coil 1.
The ignition signal G is turned to an L level when the primary current i1 reaches a sufficient current value at a timing corresponding to an ignition timing. With this operation, the power transistor 6 is turned off and the primary current i1 is shut off.
The shut-off of the primary current i1 causes magnetic energy accumulated in the core when the primary current i1 is fed, to be induced in the secondary coil 2 and output from the one end of the secondary coil 2 as the high-tension secondary voltage V2.
When the secondary voltage V2 reaches the breakdown voltage of the ignition plug 5, the ignition plug 5 starts discharging and the discharge current i2 starts to flow.
That is, the large capacitive discharge current iC instantly flows through the peripheral floating capacitive component (normally generated around the electrical line or terminal) of the ignition plug 5 and successively the inductive discharge current iL flows while being gradually reduced while the ignition plug 5 continuously discharges (the secondary voltage V2 is unchanged). With this operation, a discharge spark is generated at a predetermined ignition timing so that ignition is carried out by firing mixed gas in a cylinder.
At the time, the ignition plug 5 acts as a noise generating source and supplies a noise signal caused by the capacitive discharge current iC to the ignition coil 4 and a circuit including the ECU 10.
The noise signal influences the power transistor 6 and other circuit devices as, for example, radiant noise and radiation noise and increases faulty operation and radio noise.
A noise signal caused by the capacitive discharge current iC is superposed with the primary low-tension wiring of the ignition coil 4 through the magnetic coupling component and the capacitive coupling component C4 between the primary coil 1 and the secondary coil 2 and influences the power transistor 6 and the other circuit devices as line noise.
Further, the noise signal is superposed with the line of the ignition signal G through the capacitive coupling component C7 between the collector and base of the power transistor 6 and influences the other circuit devices including elements in the ECU 10.
In particular, although an arrangement in which the ignition coil 4 accommodates the power transistor 6 integrally therewith has been recently employed, since a wiring between the ignition coil 4 and the power transistor 6 is short in this case, a noise signal is less damped to increase influence caused by the superposition of noise as described above.
Likewise, although an arrangement in which the ignition plug 5 is directly connected to the ignition coil 4 has been employed to reduce the size of an ignition apparatus, since noise is not damped by a high-tension cable and the like in this case, influence due to the above superposition of noise is increased.
As described above, since the conventional ignition coil for an internal combustion engine does not take any measure against the capacitive discharge current iC generated at the beginning of the discharge current i2, the conventional ignition coil has a problem that the other circuit devices including the power transistor 6 and ECU 10 are liable to be faulty in operation by the influence of a superposed noise signal due to the capacitive discharge current iC.